1. Field of the Invention
The present invention relates to an electroluminescent device, particularly to an organic electroluminescent device reliably receiving driving voltage from a voltage source, and a method of driving the same.
2. Description of the Related Art
Recently, there have been active efforts to develop various display devices in which the cumbersome weight and volume of the cathode ray tube are reduced. Liquid crystal display (LCD), field emission display (FED), plasma display panel (PDP), and electroluminescent device (EL) are the kinds of display device.
PDP is most advantageous to large screen because the structure and manufacturing method are relatively simple. However, PDP has disadvantages that the emitting efficiency and brightness are low, and the consumption power is high.
The demand of LCD has been increased, as LCD is mainly used in the display device of laptop computer. However, LCD is difficult to use for large screen because it is manufactured in semiconductor process. Also, LCD is not self-emitting device, and thus needs extra light source. Due to the light source, LCD's consumption power is disadvantageously high. Moreover, LCD loses much light for optical devices, for example, polarizing filter, prism sheet, diffusion sheet, etc., and has another shortcoming that the angle of vision is narrow.
EL is classified into inorganic electroluminescent device and organic electroluminescent device. EL has advantages such as high speed, good emitting efficiency, high brightness, and wide angle of vision. Organic electroluminescent device can display the picture with tens of thousands of high brightness [cd/m2] at about 10V of voltage, and is applied to most commercial EL.
FIG. 1 is a diagram of a related-art organic electroluminescent device. FIG. 2 is a timing diagram showing scan line signals and data current applied to the organic electroluminescent device of FIG. 1. FIG. 3 is a timing diagram showing delay of replying time of a related-art organic electroluminescent device. FIG. 4 is a diagram showing a data pulse applied to a related-art organic electroluminescent device. And, FIG. 5 is a diagram showing drop of driving voltage according to a pre-charge current of FIG. 4.
In FIG. 1 and FIG. 2, the organic electroluminescent device includes a panel 20, a scan driving circuit 24, and a data driving circuit 22.
The panel 20 includes a plurality of pixels 10 formed on an area crossing over data lines (from DL1 to DLm) and scan lines (from SL1 to SLn).
The scan driving circuit 24 applies scan signals (SCAN) to the scan lines (from SL1 to SLn). The data driving circuit 22 applies data current (Id) to the data lines (from DL1 to DLm).
Each pixel 10 includes a red sub-pixel 10A, a green sub-pixel 10B, and a blue sub-pixel 10c. 
The anode of the red, green and blue sub-pixels 10A, 10B and 10C is connected to the data lines (from DL1 to DLm), and the cathode is connected to the scan lines (from SL1 to SLn). The red, green, and blue sub-pixels 10A, 10B and 10C emit light during low logic time of the scan signal (SCAN) applied to the scan lines (from SL1 to SLn) when the data current (Id) is applied to the data lines (from DL1 to DLm) as shown in FIG. 2.
That is, when the data current (Id) is applied to the red, green and blue sub-pixels 10A, 10B and 10C, the organic electroluminescent device realizes colored picture to one pixel 10 by combination of the red, green and blue sub-pixels 10A, 10B and 10C through emitting in brightness proportional to the current applied to the red, green and blue sub-pixels 10A, 10B and 10C.
However, real data current (Id) applied to the pixels 10 is smaller than the current applied from the data driving circuit 22 by resistance of the data lines (from DL1 to DLm) and capacitance of the pixels 10 as shown in FIG. 3. Also, the organic electroluminescent device has low brightness and long responsive time (RT) because emitting is delayed as much as the period of time that current is charged to the pixels 10.
Thus, as shown in FIG. 4, a pre-charge current (Ipd) is also applied to the organic electroluminescent device, besides the data current (Id). The pre-charge current (Ipd) is applied to the red, green and blue sub-pixels 10A, 10B and 10C during a pre-charge time (PT) before the data current (Id) is applied to the pixels 10.
Generally, the pre-charge current (Ipd) is ten times as much as the data current (Id). Therefore, the driving circuit of the organic electroluminescent device has to apply a lot of current to the pixels during the pre-charge time (PT).
If too high pre-charge current (Ipd) is applied to the pixels 10, the driving circuit of the organic electroluminescent device cuts off the driving voltage (V) applied from a voltage source (not shown).
In detail, the driving circuit drives the organic electroluminescent device below a prescribed current by receiving a prescribed driving voltage (V) from the voltage source. If high current like the pre-charge current (Ipd) is applied to the organic electroluminescent device at the same time, voltage drop (V_Drop) is occurred in the driving voltage (V) applied to the organic electroluminescent device, as shown in FIG. 5. And, the dropped voltage (V_Drop) is transmitted to a power driving circuit (not shown) which controls power of the organic electroluminescent device.
At this time, the power driving circuit recognizes the dropped voltage (V_Drop) as the driving voltage (V) applied from voltage source to the organic electroluminescent device. And, the power driving circuit compares the dropped voltage (V_Drop) with a critical value of the driving voltage (V) stored in memory (not shown). If the dropped voltage (V_Drop) is less than the critical value of the driving voltage (V), the power driving circuit cuts off the driving voltage (V) applied from the voltage source to the organic electroluminescent device because the power driving circuit recognizes that voltage of the voltage source for driving the organic electroluminescent device is short.
Therefore, the driving voltage (V) cannot be reliably applied to the organic electroluminescent device because of very high pre-charge current (Ipd) applied at once.